The present invention relates to the preparation and observation of topographic sections of semiconductor devices which are highly integrated on a wafer of Si crystal, etc., by an ultra-fine process. The apparatus can be used for evaluation of the LSI fabrication process so as to find problems in the semiconductor device fabrication process.
In order to increase the integration density of large scale integrated circuits (LSI), the individual circuit elements are advantageously made small, and integrated circuits having three dimensional structures have been proposed, including multi-layer lead patterns and trench capacitive elements. As a result, the semiconductor fabrication process becomes more and more complicated and requires several tens to several hundreds of fabrication steps.
Conventionally, a scanning electron microscope is used for topographic surface observation to evaluate the fabrication steps. However, an evaluation of internal structure is necessary for failure analysis of a three dimensional structure. In this regard, since non-destructive surface observation by the scanning electron microscope is not highly effective, observation of topographic sections and profiles has been undertaken by mechanically cutting or scribing a sample. Such mechanical method is not practical for topographic observation of a particular tiny region of interest. However, observation of particular tiny regions of an LSI device is required for failure analysis. For instance, numerous contact holes are formed on a single LSI chip so as to interconnect vertically between multilayered lead patterns, and only one or a few of these is normally defective. Therefore, an analysis of such defective contact hole is quite difficult by conventional mechanical cutting, polishing and etching methods in view of likely positional errors.
FIG. 1 is a top plan view of a contact hole region on an LSI device. This region contains aluminum lead strips 21, a crossing polysilicon lead strip 22 and a contact hole 23 therebetween. In order to undertake topographic observation of the region containing these elements, the sample is cut along a particular line 24 as shown by a chain line in FIG. 1. FIG. 2 is a sectional view taken along this particular line 24. This section contains the upper aluminum lead strip 21, the lower polysilicon lead strip 22 which is electrically connected to the upper aluminum lead strip 21 through contact hole 23, a protective insulating film 25, made of such as silicon nitride, and a silicon substrate 26.
For preparation of the exposed section, the surface of the sample is selectively ion-sputtered by a scanning ion microscopic device to form a crater as defined by chain line 24 of FIG. 1. Then a microscope stage carrying the sample is inclined so as to permit observation of the section thus exposed, as shown in FIG. 2. However, according to such preparation and observation method, in order to observe the section and profile step by step, the sample stage must be angularly displaced several times between a preparation position which is normally horizontal, and an observation position, inclined at 45.degree. to 60.degree. with respect to a horizontal plane, so as to switch between the preparation procedure and the observation procedure for the particular section, thereby causing mechanical error and requiring complicated operations.
Further, since the section cannot be observed in situ during the preparation of the section or during continuous etching of the sample surface, microscopic foreign substances and abnormal micro structures which would be a cause of defects of the contact hole may inadvertently escape the observation.